This invention relates to forming a web from staple fibers, and more particularly to method and means for forming a web of such fibers, such as of wool, cotton, man-made materials and so forth, which has enhanced uniformity of properties both in the transverse and longitudinal directions. Yet more particularly, the invention relates to forming such a web with what is known in the art as a random orientation distribution of staple fibers, and which possesses not only an enhanced uniformity of properties such as tear strengths in the transverse and longitudinal directions of the web but in fact enhanced isotropy, which is to say enhanced uniformity of properties along all axes of direction of the web.
The present invention employs centrifugal, dynamic web forming. A means and method therefor known in the art are as disclosed in U.S. Pat. No. 4,279,060 of July 21, 1981. Generally, one uses a carding action to disentangle the staple fibers of fibrous flocks and form a web of a layer of such fibers oriented such that their axes extend more parallel one another and in the direction of fiber flowpath in the process machinery. The resultant web is removed from the carding device and is subjected to an accumulator, or bunching or jamming action to increase its depth to provide a more dense web of greater than one-fiber thickness, and thereafter is subjected to centrifugal aerodynamic forces, so called free web formation, and then doffed from the apparatus. The doffed web then is suitable for use in producing nonwoven fabrics. Depending upon the degree of bunching and after-treatment of the webs produced, the webs are suitable for producing the so-called light-weight nonwoven fabrics, usually of a weight within the range of 8 to 25 grams per square meter of fabric surface area; similarly, one as desired may form medium weight from 25 up to 70 grams per square meter, or heavy weight from 70 up to 250 grams per square meter, nonwoven fabrics.
The means of U.S. Pat. No. 4,279,060 is particularly instructive in understanding the foregoing method. There one employs the well known carding cylinder and doffer; however, interposed therebetween is the centrifugal, aerodynamic web former in the form of an intermediate toothed roller having a set of protruding teeth with the front rake thereof being from zero to just a few angular degrees, whereby the extension of the front rake of the toothing theoretically passes through the center point of the circular path formed by the roller surface bearing the toothing. Coacting with the intermediate roller is a guide plate which generally follows the curvature of the roller but is spaced somewhat away from the roller's surface along the flowpath of material carried by the roller such that the spacing progressively diminishes downstream so that, in an example, at the entry to the space between the teeth and guideplate the spacing may be 4 to 6 millimeters and diminish to a spacing of only 1 millimeter at the region of exiting. This produces what is now known as the accumulating, bunching or jamming action. When the bunched fibrous layer or web leaves the teeth of the intermediate roller, by centrifugal force so to speak, it is flung a short distance, there said to be in example 8 to 12 millimeters, through what is called a free zone passing therethrough in aerodynamic flight until it encounters the teeth of the doffer roll. Evidentally, during such aerodynamic flight the web is freed from the jamming effect and evens out to form the random non-woven web of staple fibers, as a non-woven fabric.
It has been found, however, that the fabric produced shows some anisotropy, though superior to many other prior art nonwovens.